This invention relates generally to filters and more particularly to a notch filter circuit apparatus.
In many circuits it is desirable to operate the circuit so that one frequency signal is highly attenuated, while a desired frequency signal is left unattenuated. A circuit input, for example, may include not only a fundamental frequency signal, but may also include second, third, fourth, and higher harmonic frequency signals. In some circuit implementations it may be required to pass the fundamental frequency signal while blocking a specific harmonic signal. A notch, or bandstop, filter is the most appropriate filter to meet this requirement. A bandpass filter that discriminates against a wide range of frequency signals outside the passband may not provide the desired results.
Notch filters are often realized using distributed transmission line stubs, which can occupy significant substrate space. In conventional coplanar waveguide circuits, a notch filter may be created by symmetrically placing shunt stubs on opposite sides of the coplanar waveguide line. Conventional methods for reducing stub length, and therefore scarce substrate space, include using bent shunt stubs, meander structures, or capacitive loading. Notch filters employing these methods may be difficult to control over a broad frequency band or in more than one narrow frequency band of interest.
According to one embodiment of the invention, a notch filter circuit includes a coplanar waveguide that is located on a silicon substrate and at least one shunt stub bent at an angle to the coplanar waveguide. The notch filter circuit further includes at least one capacitor bridging at least one discontinuity of the shunt stub.
Some embodiments of the invention provide numerous technical advantages. Other embodiments may realize some, none, or all of these advantages. For example, according to one embodiment, a notch filter circuit utilizes at least one metal-insulator-metal capacitor in place of an air bridge or wire-bond to reduce the physical size of the notch filter. In some embodiments, the metal-insulator-metal capacitor also provides coplanar waveguide ground equalization. In addition the notch filter circuit may be implemented on a high-resistivity silicon substrate. In some embodiments, multiple metal-insulator-metal capacitors are located at specific positions along the length of stub to allow the filter pass-band and stop-band to be properly selected.